1. Field of the Invention
The present invention relates to a solid state image pickup device constituted by an image pickup element in which pixels are arranged in, e.g., the form of a matrix, a driving method therefor, and a camera comprising an optical system and a solid state image pickup device.
2. Description of the Related Art
As a solid state image pickup device, there is a solid state image pickup device constituted by a so-called MOS type or CMOS type image pickup element in which each unit pixel is constituted by a having MOS transistor, a signal charge accumulated on a pixel is read by photoelectric conversion, and the signal charge is converted into a voltage to output the voltage.
In the MOS type or CMOS type image pickup element, for example, a MOS transistor is used as a switching element for selecting a pixel and a switching element for reading a signal charge.
A MOS transistor or a CMOS transistor is used in a horizontal scanning circuit or a vertical scanning circuit, and can be advantageously manufactured with an arrangement in series with a switching element.
In a conventional so-called X-Y address type MOS or CMOS image pickup element in which unit pixels are secondarily arranged in row and column in a two-dimensional fashion in order to perform an electronic shutter, unnecessary signal charges are reset (exhausted) from pixels of one row to a signal line in a horizontal blanking period in which signal charges are not read.
FIG. 1 is a schematic view showing the arrangement of a conventional solid state image pickup device having a CMOS image pickup element.
This solid state image pickup device 50 is constituted by an image pickup region formed by arranging a plurality of unit pixels 60, each of which is constituted by a photodiode 51 for performing photoelectric conversion and a vertical selection switch 52 for selecting a pixel, in the form of a matrix, a vertical scanning circuit 53 for outputting a vertical scanning pulse φVm to a vertical selection line 54 to which control electrodes of the vertical selection switches 52 are commonly connected at each row, a vertical signal line 55 to which main electrodes of the vertical selection switches 52 of each column are commonly connected at each row, a horizontal switch 56 connected to the vertical signal line 55 and a horizontal signal line 58, a horizontal scanning circuit 57 connected to the control electrode of the horizontal switch 56, and an amplifier 59 connected to the horizontal signal line 58.
As a basic operation of the solid state image pickup device 50, a signal charge photoelectrically converted by the photodiode 51 is read to the vertical signal line 55 through the vertical selection switch 52 controlled by the vertical scanning circuit 53. In a horizontal video period HA, the signal charge read to the vertical signal line 55 is sequentially output to the horizontal signal line 58 through the horizontal switch 56 controlled by the horizontal scanning circuit 57, and is converted into a signal voltage by the amplifier 59 connected to the horizontal signal line 58 to output the signal voltage.
In the conventional solid state image pickup device 50, when an electronic shutter operation is to be performed, the vertical signal line 55 is also used to exhaust an unnecessary signal charge. For this reason, in a horizontal blanking period HBLK in which no signal is output, the unnecessary signal charges are exhausted.
At this time, at a timing shown in FIG. 2, a vertical scanning pulse φVm+k of the vertical selection line 54 at an(m+k)th row rises in the horizontal blanking period HBLK in a certain horizontal scanning period TH, and signal charges are exhausted from the photodiodes 51 of the corresponding row.
On the other hand, a vertical scanning pulse φVm of the mth row rises in a horizontal video period HA of the same horizontal scanning period TH, signal charges are read from the photodiodes 51 to the vertical signal line 55, and a signal is output from the solid state image pickup device 50.
By the way, a shutter speed of an electronic shutter, i.e., a time corresponding to the accumulation time of a pixel is determined by a time from an exhaust time of signal charges to a read time of a signal charge. For this reason, when a horizontal scanning period, a horizontal blanking period, and the number of horizontal pixels are represented by TH, HBLK, and N, respectively, the accumulation time of a pixel at one end in the horizontal direction output by a horizontal scanning pulse φH1 is k×TH, and the accumulation time of a pixel at the other end in the horizontal direction output by a horizontal scanning pulse φHN is k×TH+(TH−HBLK). As a result, the accumulation times of the pixels on the right and left sides in the horizontal direction are different from each other.
More specifically, the accumulation time of a pixel output by a horizontal scanning pulse φHn changes in proportion to a timing at which that pulse is output.
When an electronic shutter operation is performed in a conventional solid state image pickup device as described above, unnecessary charges are simultaneously exhausted in the respective rows, reading of the signal charge are sequentially performed according to scanning, and the accumulation times in the respective rows change in the range of k×TH to k×TH+(TH−HBLK). This change especially, adversely affects a solid state image pickup device in which a shutter speed is set to be high and the value of k is small.
As described above, in a conventional solid state image pickup device, with respect to a pixel row which is reset in the horizontal blanking period HBLK, the accumulation time of a pixel read first by the horizontal scanning is different from the accumulation time of a pixel finally read by the horizontal scanning. The difference between the accumulation times is almost a time corresponding to a horizontal scanning period.
When the accumulation time of the pixel, i.e., a shutter speeds is sufficiently long, the difference between the accumulation times described above can be neglected. However, when the shutter speed becomes as long as the horizontal scanning period, the difference between the accumulation times appears as shading in a line direction (row direction).